Gentlemen, thank you. My bias is: I'd rather pay $650-700 for a perfect pair than $399 for a pair with a visible issue on the vamp.
I'd happily keep them if all I had to do was keep a little polish on the vamp. But if it lightens "disproportionately faster" than the blotch, then I would probably return them (and the seller does accept returns).
Is it only going to get worse?
Is this a keep or return situation?
Your first paragraph is spot on.
That spot will not go away.